In recent years, attention has been focused on a plasma thruster as a thruster for space rockets. In this plasma thruster, gas such as xenon working as a propellant (thrust gas) is ionized and made into charged particles. Thereafter, the charged particles are affected by electromagnetic force and expansive force so that the charged particles are thrusted with high efficiency compared to a thruster for space rockets in the related arts. The following apparatus has been suggested as an example of a plasma generation apparatus relating to such a plasma thruster. That is, an apparatus in which light from plasma generated in a discharge vessel is detected so as to measure temporal changes in ion density of discharge plasma (For example, Japanese Patent Publication No. 2000-269191).